I found this on Sweedish Bricks and it described my problem exactly. I took out and cleaned the oil strained and changed the oil.
Intermittent Shift Failure: Clogged Filter
[Query:] After starting, everything goes well, the transmission shifts, but in a short while, suddenly, the transmission becomes disconnected, losing traction; moving, I accelerate and the motor increases revolutions but the car behaves as though it were in neutral. I must stop the motor, wait a moment and repeat the operation. While the problem is occurring, if I accelerate in neutral I hear a slight buzzing noise of gears even though the transmission has not engaged. The oil is new. I changed the kickdown cable. [Response: Abe Crombie] The things you list sound like a stopped up filter inside transmission pan. Did the pan get removed and the filter inspected? The filter is a fine metal mesh strainer and can be cleaned in most cases. I didn't read your previous post of a month ago so I do not know how this started but using shop clothes to wipe off things inside transmission or to wipe the pan when it is off, can introduce lint that the transmission filter will catch when it is running. The debris on filter then starves the transmission pump for oil. The transmission pump will whine when operating with excessive vacuum on its inlet due to a plugged filter. When you stop and shut down engine, the lint falls off the filter and it will work again for a period of time until the lint is sucked up onto filter once again.